In recent years, display technology has been developed rapidly, for example, TFT technology has been developed from original a-Si (amorphous silicon) TFT to current LTPS (low temperature poly-silicon) TFT, MILC (metal-induced lateral crystallization) TFT, oxide TFT, etc. While luminescence technology has also been developed from original LCD (liquid crystal display) and PDP (Plasma Display Panel) to current OLED (organic light-emitting diode), AMOLED (Active Matrix/Organic Light Emitting Diode), etc. OLED display is a new generation of display device, compared with LCD display, it has many advantages, such as self-luminance, quick response, wide viewing angle, etc., and can be used for flexible display, transparent display, 3D (three-dimensional) display, etc. But as to both LCD and OLED display, for every pixel, a switching-TFT for controlling the pixel is required to be provided, which can independently control every pixel through a driving circuit without causing crosstalk or other impacts on other pixels.
Oxide TFT which is widely used at present uses oxide semiconductor as an active layer, has characteristics of high mobility, large on-state current, excellent switching characteristics and better uniformity, and can be used in an application requiring quick response and large current, such as in a display with high-frequency, high resolution and large size, an OLED display, etc.
In the prior art, five mask (exposure) processes are generally required in fabricating an oxide TFT backplane, respectively for forming a gate line and a gate, an active layer, an etch stop layer, a source and a drain, a passivation layer and a via hole. Research shows that film performance and complicated preparing process of the etch stop layer greatly affects oxide semiconductor, which usually results in threshold voltage (Vth) drift, small Ion (on-state current)/Ioff (off-state current) and poor thermal stability of TFT. In addition, fabrication of the etch stop layer needs a separate mask process, and fabrication cost is increased accordingly.